Method for conducting digital interface and baseband circuitry tests using digital loopback

ABSTRACT

In a mobile device having a primary baseband circuit and a secondary baseband circuit and an interface between the primary baseband circuit and a secondary baseband circuit, a method for testing the interface and primary and secondary baseband circuits comprising the steps of: setting the secondary baseband circuit into a loopback mode; sending a test signal from the primary baseband circuit to the secondary baseband circuit; receiving at the primary baseband circuit a second signal, the second signal being the first signal looped back from the secondary baseband circuit; and comparing the second signal with an expected result.

FIELD OF THE APPLICATION

The present application deals with a method for testing an interface andbaseband circuitry and, in particular, to testing a digital interfaceusing a test tone, which could be comprised of multiple frequency tonesor a single tone generated by one baseband chip while the other basebandchip interface is configured in a loopback mode.

BACKGROUND

Many modern mobile devices include two audio baseband chips. These aretypically used for various communication means by the mobile device. Inone example, such communication means could include a radio frequencycommunication to communicate over a wireless network, such as a Mobitex™mobile communication system, a DataTAC™ mobile communication system,GPRS network, UMTS network, EDGE network, or CDMA network. A secondarybaseband circuit could be used for communications for short-rangesystems including a Bluetooth™ system.

One problem with present devices with two audio baseband chips is theinability to test the interface between the audio baseband chips.Generally, hardware needs to be added to a circuit board in order tofacilitate the testing of these audio baseband chips. Further, expensivetest equipment is required for this testing.

Further, to properly test the interface would require the enabling ofthe radio for both the audio baseband chips which requires the settingup of radio test equipment and acoustic test equipment for generatingand analyzing audio test signals.

SUMMARY

The present method is used to verify two audio baseband circuits and thedigital interface between the two audio baseband circuits withoutrequiring any external test equipment. In a preferred embodiment, onebaseband circuit is a mobile station digital baseband chip and thesecond baseband circuit is a Bluetooth™ baseband chip. The interfacebetween the two audio baseband chips is a codec PCM interface, however,as will be appreciated by those skilled in the art, other digital chipsand digital interfaces could be used with the present method and theexample of a mobile station baseband circuit and a Bluetooth™ basebandchip with a PCM interface is, in no way, meant to limit the scope of thepresent method.

In one embodiment, the present method uses a Bluetooth™ protocol radiotest command to configure the Bluetooth™ audio baseband circuit into adigital loopback mode. From the mobile station audio baseband circuit, asingle tone or dual tone multiple frequency (DTMF) test signal isgenerated and transmitted internally to the Bluetooth™ audio basebandcircuit over the codec PCM transmit interface. The Bluetooth™ audiobaseband circuit will loop back the test signal to the mobile stationaudio baseband circuit over the codec PCM receive interface. The mobilestation audio baseband circuit will detect the test signal and will usesoftware to read a specific register, which stores the looped back testtone data and compare the data with the expected result. This willverify the interface and audio baseband circuits.

The present application therefore provides, in a mobile device having afirst audio baseband circuit and a second audio baseband circuit and aninterface between the first audio baseband circuit and a second audiobaseband circuit, a method for internal verification of the interfaceand first and second audio baseband circuits comprising the steps of:setting the second audio baseband circuit into a loopback mode; sendinga test signal from the first audio baseband circuit to the second audiobaseband circuit via the interface; receiving at the first audiobaseband circuit a second signal via the interface, the second signalbeing the test signal looped back from said second audio basebandcircuit; and comparing the second signal to the original test signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The present method will be better understood with reference to thedrawings in which:

FIG. 1 is a block diagram of a mobile station with two digital basebandchips;

FIG. 2 is a flow chart of a method of testing the digital interface.

FIG. 3 is a block diagram of an alternative embodiment of the presentmethod with an analog loopback instead of a digital loopback; and

FIG. 4 shows a block diagram of a communications system, including amobile station upon which the present method can be implemented; and

FIG. 5 shows a block diagram of a mobile station upon which the presentmethod can be implemented.

DETAILED DESCRIPTION OF THE DRAWINGS

The present method is used for the internal verification of an interfacebetween two audio baseband circuits. Since the interface can be verifiedinternally, this reduces the need for external test equipment andreduces the external components needed on a circuit board for a mobilestation.

Reference is now made to the drawings. FIG. 1 shows a mobile station 10.Mobile station 10 according to the present method includes two digitalbaseband circuits which are labeled as primary baseband circuit 12 andsecondary baseband circuit 14. In one embodiment, the primary basebandcircuit is a combination of the radio frequency baseband chip and adigital audio baseband chip for the primary communication of a mobilestation. Secondary baseband circuit 14 is preferably comprised of asecondary communication baseband circuit such as a bluetooth™ basebandchip which includes both the radio frequency and digital audio basebandchip in one. Such chips are known in the art and are made, for example,by Qualcomm.

Mobile station 10 includes numerous other components besides primarybaseband circuit 12 and secondary baseband circuit 14, and these arespecified in more detail below with reference to FIG. 4.

Mobile station 10 includes an interface 16 between primary basebandcircuit 12 and secondary baseband circuit 14. In one embodiment,interface 16 includes four lines which are comprised of a transmit line,a receive line, and two clock lines.

Primary baseband circuit 12 preferably includes a dual-tone multiplefrequency (DTMF) module 20. A DTMF module 20 is used by primary basebandcircuit 12 for touchtone dialing. It generates a combination of twotones where one tone is a low frequency and the other a high frequency.A DTMF module 20 exists in most primary baseband circuits 12.

Secondary baseband circuit 14 includes a digital interface module 24which is generally used by secondary baseband circuit 14 to transmit asignal received over the interface 16 to the outside world. In apreferred embodiment, digital interface module 24 is a pulse codemodulation (PCM) module.

Digital interface module 24 allows the secondary baseband circuit 14 tobe configured into a PCM loop back mode. Basically, this causes signalsreceived at interface 16 to be looped back and sent to the originator.The signal sent to secondary baseband circuit 14 can be considered atest signal and the signal received from secondary baseband circuit 14can be considered a second signal.

Reference is now made to FIG. 2.

A method of testing according to the present application is described.In step 40, secondary baseband circuit 14 is configured so that digitalinterface 24 is put into a loopback mode.

Next, in step 42, DTMF module 20 generates a tone that in step 44 istransmitted to the secondary baseband circuit 14. Once the signal isreceived at secondary baseband circuit, it is looped back throughdigital interface module 24 to primary baseband circuit 12 in step 46.

In step 48, the primary baseband circuit 12 receives the loopbacksignal, and detects the DTMF signal. These results are put into aregister 18 and test software is then used to read register 18 andcompare data within that register with the expected result. Thiscomparison checks whether the signal level and frequency are at theexpected values.

Accordingly, the present method allows for the testing of the interfacebetween the primary and the secondary baseband circuits 12 and 14respectively by generating a signal at the primary baseband circuit 12,sending it over the interface 16 to secondary baseband circuit 14 whereit is looped back through PCM loopback mode back to primary basebandcircuit 12. At this point, it is tested to see whether it matches whatthe expected result should be.

Since the present method is completely internal within mobile station10, external equipment is therefore not needed, saving time and expense.Further, space on the circuit board is saved by not requiring externalcomponents on the board for test purposes.

Reference is now made to FIG. 3. In an alternative configuration,loopback could occur in analog module 26 and the signal could be merelypassed through digital module 24 within secondary baseband circuit 14.In this case, digital interface module 24 would convert the signal to ananalog signal and the analog module 26 would merely loop back to thedigital module 24 where the signal would again be converted to a digitalsignal and sent back over interface 16 to primary baseband circuit 12where the signal would be stored in a register 18. Accordingly, loopbackcould therefore occur in the analog portion of secondary basebandcircuit 14.

The present method therefore verifies the digital interface between theprimary and secondary baseband circuits without using any external testequipment. As will be appreciated by those skilled in the art, thisgenerally comprises the voice path for signals.

Reference is now made to FIG. 4. FIG. 4 is a block diagram of acommunication system 100 which includes a mobile station 102 whichcommunicates through a wireless communication network 104. Mobilestation 102 preferably includes a visual display 112, a keyboard 114,and perhaps one or more auxiliary user interfaces (UI) 116, each ofwhich is coupled to a controller 106. Controller 106 is also coupled toradio frequency (RF) transceiver circuitry 108 and an antenna 110.

Typically, controller 106 is embodied as a central processing unit (CPU)which runs operating system software in a memory component (not shown).Controller 106 will normally control overall operation of mobile station102, whereas signal processing operations associated with communicationfunctions are typically performed in RF transceiver circuitry 108.Controller 106 interfaces with device display 112 to display receivedinformation, stored information, user inputs, and the like. Keyboard114, which may be a telephone type keypad or full alphanumeric keyboard,is normally provided for entering data for storage in mobile station102, information for transmission to network 104, a telephone number toplace a telephone call, commands to be executed on mobile station 102,and possibly other or different user inputs.

Mobile station 102 sends communication signals to and receivescommunication signals from network 104 over a wireless link via antenna110. RF transceiver circuitry 108 performs functions similar to those ofa radio network (RN) 128, including for example modulation/demodulationand possibly encoding/decoding and encryption/decryption. It is alsocontemplated that RF transceiver circuitry 108 may perform certainfunctions in addition to those performed by RN 128. It will be apparentto those skilled in art that RF transceiver circuitry 108 will beadapted to particular wireless network or networks in which mobilestation 102 is intended to operate.

Mobile station 102 includes a battery interface 122 for receiving one ormore rechargeable batteries 124. Battery 124 provides electrical powerto electrical circuitry in mobile station 102, and battery interface 122provides for a mechanical and electrical connection for battery 124.Battery interface 122 is coupled to a regulator 126 which regulatespower to the device. When mobile station 102 is fully operational, an RFtransmitter of RF transceiver circuitry 108 is typically turned on onlywhen it is sending to network, and is otherwise turned off to conserveresources. Similarly, an RF receiver of RF transceiver circuitry 108 istypically periodically turned off to conserve power until it is neededto receive signals or information (if at all) during designated timeperiods.

Mobile station 102 operates using a memory module 120, such as aSubscriber Identity Module (SIM) or a Removable User Identity Module(R-UIM), which is connected to or inserted in mobile station 102 at aninterface 118. As an alternative to a SIM or an R-UIM, mobile station102 may operate based on configuration data programmed by a serviceprovider into an internal memory which is a non-volatile memory. Mobilestation 102 may consist of a single unit, such as a data communicationdevice, a cellular telephone, a multiple-function communication devicewith data and voice communication capabilities, a personal digitalassistant (PDA) enabled for wireless communication, or a computerincorporating an internal modem. Alternatively, mobile station 102 maybe a multiple-module unit comprising a plurality of separate components,including but in no way limited to a computer or other device connectedto a wireless modem. In particular, for example, in the mobile stationblock diagram of FIG. 4, RF transceiver circuitry 108 and antenna 110may be implemented as a radio modem unit that may be inserted into aport on a laptop computer. In this case, the laptop computer wouldinclude display 112, keyboard 114, and one or more auxiliary UIs 116,and controller 106 may remain within the radio modem unit thatcommunicates with the computer's CPU or be embodied as the computer'sCPU. It is also contemplated that a computer or other equipment notnormally capable of wireless communication may be adapted to connect toand effectively assume control of RF transceiver circuitry 108 andantenna 110 of a single-unit device such as one of those describedabove. Such a mobile station 102 may have a more particularimplementation as described later in relation to mobile station 202 ofFIG. 5.

Mobile station 102 communicates in and through wireless communicationnetwork 104. In the embodiment of FIG. 4, wireless network 104 is aThird Generation (3G) supported network based on Code Division MultipleAccess (CDMA) technologies. In particular, wireless network 104 is aCDMA2000 network which includes fixed network components coupled asshown in FIG. 3. Wireless network 104 of the CDMA2000-type includes aRadio Network (RN) 128, a Mobile Switching Center (MSC) 130, a SignalingSystem 7 (SS7) network 140, a Home Location Register/AuthenticationCenter (HLR/AC) 138, a Packet Data Serving Node (PDSN) 132, an IPnetwork 134, and a Remote Authentication Dial-In User Service (RADIUS)server 136. SS7network 140 is communicatively coupled to a network 142(such as a Public Switched Telephone Network or PSTN), whereas IPnetwork is communicatively coupled to a network 144 (such as theInternet).

During operation, mobile station 102 communicates with RN 128 whichperforms functions such as call-setup, call processing, and mobilitymanagement. RN 128 includes a plurality of base station transceiversystems that provide wireless network coverage for a particular coveragearea commonly referred to as a “cell”. A given base station transceiversystem of RN 128, such as the one shown in FIG. 4, transmitscommunication signals to and receives communication signals from mobilestations within its cell. The base station transceiver system normallyperforms such functions as modulation and possibly encoding and/orencryption of signals to be transmitted to the mobile station inaccordance with particular, usually predetermined, communicationprotocols and parameters, under control of its controller. The basestation transceiver system similarly demodulates and possibly decodesand decrypts, if necessary, any communication signals received frommobile station 102 within its cell. Communication protocols andparameters may vary between different networks. For example, one networkmay employ a different modulation scheme and operate at differentfrequencies than other networks. The underlying services may also differbased on its particular protocol revision.

The wireless link shown in communication system 100 of FIG. 4 representsone or more different channels, typically different radio frequency (RF)channels, and associated protocols used between wireless network 104 andmobile station 102. An RF channel is a limited resource that must beconserved, typically due to limits in overall bandwidth and a limitedbattery power of mobile station 102. Those skilled in art willappreciate that a wireless network in actual practice may includehundreds of cells depending upon desired overall expanse of networkcoverage. All pertinent components may be connected by multiple switchesand routers (not shown), controlled by multiple network controllers.

For all mobile station's 102 registered with a network operator,permanent data (such as mobile station 102 user's profile) as well astemporary data (such as mobile station's 102 current location) arestored in a HLR/AC 138. In case of a voice call to mobile station 102,HLR/AC 138 is queried to determine the current location of mobilestation 102. A Visitor Location Register (VLR) of MSC 130 is responsiblefor a group of location areas and stores the data of those mobilestations that are currently in its area of responsibility. This includesparts of the permanent mobile station data that have been transmittedfrom HLR/AC 138 to the VLR for faster access. However, the VLR of MSC130 may also assign and store local data, such as temporaryidentifications. Mobile station 102 is also authenticated on systemaccess by HLR/AC 138. In order to provide packet data services to mobilestation 102 in a CDMA2000-based network, RN 128 communicates with PDSN132. PDSN 132 provides access to the Internet 144 (or intranets,Wireless Application Protocol (WAP) servers, etc.) through IP network134. PDSN 132 also provides foreign agent (FA) functionality in mobileIP networks as well as packet transport for virtual private networking.PDSN 132 has a range of IP addresses and performs IP address management,session maintenance, and optional caching. RADIUS server 136 isresponsible for performing functions related to authentication,authorization, and accounting (AAA) of packet data services, and may bereferred to as an AAA server.

Wireless communication network 104 also includes a Push-to-talk overCellular (PoC) server 137 which may be coupled to IP network 134. PoCserver 137 operates to facilitate PoC individual and group communicationsessions between mobile stations within network 104. A conventional PoCcommunication session involves a session connection between end users ofmobile stations, referred to as session “participants”, who communicateone at a time in a half-duplex manner much like conventionalwalkie-talkies or two-way radios.

Those skilled in art will appreciate that wireless network 104 may beconnected to other systems, possibly including other networks, notexplicitly shown in FIG. 4. A network will normally be transmitting atvery least some sort of paging and system information on an ongoingbasis, even if there is no actual packet data exchanged. Although thenetwork consists of many parts, these parts all work together to resultin certain behaviours at the wireless link.

FIG. 5 is a detailed block diagram of a preferred mobile station 202.Mobile station 202 is preferably a two-way communication device havingat least voice and advanced data communication capabilities, includingthe capability to communicate with other computer systems. Depending onthe functionality provided by mobile station 202, it may be referred toas a data messaging device, a two-way pager, a cellular telephone withdata messaging capabilities, a wireless Internet appliance, or a datacommunication device (with or without telephony capabilities). Mobilestation 202 may communicate with any one of a plurality of base stationtransceiver systems 200 within its geographic coverage area. Mobilestation 202 selects or helps select which one of base stationtransceiver systems 200 it will communicate with.

Mobile station 202 will normally incorporate a communication subsystem211, which includes a receiver 212, a transmitter 214, and associatedcomponents, such as one or more (preferably embedded or internal)antenna elements 216 and 218, local oscillators (LOs) 213, and aprocessing module such as a digital signal processor (DSP) 220.Communication subsystem 211 is analogous to RF transceiver circuitry 108and antenna 110 shown in FIG. 4. As will be apparent to those skilled infield of communications, particular design of communication subsystem211 depends on the communication network in which mobile station 202 isintended to operate.

Mobile station 202 may send and receive communication signals over thenetwork after required network registration or activation procedureshave been completed. Signals received by antenna 216 through the networkare input to receiver 212, which may perform such common receiverfunctions as signal amplification, frequency down conversion, filtering,channel selection, and like, and in example shown in FIG. 5,analog-to-digital (A/D) conversion. A/D conversion of a received signalallows more complex communication functions such as demodulation anddecoding to be performed in DSP 220. In a similar manner, signals to betransmitted are processed, including modulation and encoding, forexample, by DSP 220. These DSP-processed signals are input totransmitter 214 for digital-to-analog (D/A) conversion, frequency upconversion, filtering, amplification and transmission over communicationnetwork via antenna 218. DSP 220 not only processes communicationsignals, but also provides for receiver and transmitter control. Forexample, the gains applied to communication signals in receiver 212 andtransmitter 214 may be adaptively controlled through automatic gaincontrol algorithms implemented in DSP 220 or based on a gain parameterderived from a specific auxiliary device, as described below.

Network access is associated with a subscriber or user of mobile station202, and therefore mobile station 202 requires a memory module 262, suchas a Subscriber Identity Module or “SIM” card or a Removable UserIdentity Module (R-UIM), to be inserted in or connected to an interface264 of mobile station 202 in order to operate in the network.Alternatively, memory module 262 may be a non-volatile memory which isprogrammed with configuration data by a service provider so that mobilestation 202 may operate in the network. Since mobile station 202 is amobile battery-powered device, it also includes a battery interface 254for receiving one or more rechargeable batteries 256. Such a battery 256provides electrical power to most if not all electrical circuitry inmobile station 202, and battery interface 254 provides for a mechanicaland electrical connection for it. The battery interface 254 is coupledto a regulator (not shown in FIG. 5) which provides power V+ to all ofthe circuitry.

Mobile station 202 includes a microprocessor 238 (which is oneimplementation of controller 106 of FIG. 4) which controls overalloperation of mobile station 202. This control includes network selectiontechniques of the present application. Communication functions,including at least data and voice communications, are performed throughcommunication subsystem 211. Microprocessor 238 also interacts withadditional device subsystems such as a display 222, a flash memory 224,a random access memory (RAM) 226, auxiliary input/output (I/O)subsystems 228, a serial port 230, a keyboard 232, a speaker 234, amicrophone 236, a short-range communications subsystem 240, and anyother device subsystems generally designated at 242. Some of thesubsystems shown in FIG. 4 perform communication-related functions,whereas other subsystems may provide “resident” or on-device functions.Notably, some subsystems, such as keyboard 232 and display 222, forexample, may be used for both communication-related functions, such asentering a text message for transmission over a communication network,and device-resident functions such as a calculator or task list.Operating system software used by microprocessor 238 is preferablystored in a persistent store such as flash memory 224, which mayalternatively be a read-only memory (ROM) or similar storage element(not shown). Those skilled in the art will appreciate that the operatingsystem, specific device applications, or parts thereof, may betemporarily loaded into a volatile store such as RAM 226.

Microprocessor 238, in addition to its operating system functions,preferably enables execution of software applications on mobile station202. A predetermined set of applications which control basic deviceoperations, including at least data and voice communicationapplications, will normally be installed on mobile station 202 duringits manufacture. A preferred application that may be loaded onto mobilestation 202 may be a personal information manager (PIM) applicationhaving the ability to organize and manage data items relating to usersuch as, but not limited to, e-mail, calendar events, voice mails,appointments, and task items. Naturally, one or more memory stores areavailable on mobile station 202 and SIM 256 to facilitate storage of PIMdata items and other information.

The PIM application preferably has the ability to send and receive dataitems via the wireless network. In a preferred embodiment, PIM dataitems are seamlessly integrated, synchronized, and updated via thewireless network, with the mobile station user's corresponding dataitems stored and/or associated with a host computer system therebycreating a mirrored host computer on mobile station 202 with respect tosuch items. This is especially advantageous where the host computersystem is the mobile station user's office computer system. Additionalapplications may also be loaded onto mobile station 202 through network,an auxiliary I/O subsystem 228, serial port 230, short-rangecommunications subsystem 240, or any other suitable subsystem 242, andinstalled by a user in RAM 226 or preferably a non-volatile store (notshown) for execution by microprocessor 238. Such flexibility inapplication installation increases the functionality of mobile station202 and may provide enhanced on-device functions, communication-relatedfunctions, or both. For example, secure communication applications mayenable electronic commerce functions and other such financialtransactions to be performed using mobile station 202.

In a data communication mode, a received signal such as a text message,an e-mail message, or web page download will be processed bycommunication subsystem 211 and input to microprocessor 238.Microprocessor 238 will preferably further process the signal for outputto display 222 or alternatively to auxiliary I/O device 228. A user ofmobile station 202 may also compose data items, such as e-mail messages,for example, using keyboard 232 in conjunction with display 222 andpossibly auxiliary I/O device 228. Keyboard 232 is preferably a completealphanumeric keyboard and/or telephone-type keypad. These composed itemsmay be transmitted over a communication network through communicationsubsystem 211.

For voice communications, the overall operation of mobile station 202 issubstantially similar, except that the received signals would be outputto speaker 234 and signals for transmission would be generated bymicrophone 236. Alternative voice or audio I/O subsystems, such as avoice message recording subsystem, may also be implemented on mobilestation 202. Although voice or audio signal output is preferablyaccomplished primarily through speaker 234, display 222 may also be usedto provide an indication of the identity of a calling party, duration ofa voice call, or other voice call related information, as some examples.

Serial port 230 in FIG. 5 is normally implemented in a personal digitalassistant (PDA)-type communication device for which synchronization witha user's desktop computer is a desirable, albeit optional, component.Serial port 230 enables a user to set preferences through an externaldevice or software application and extends the capabilities of mobilestation 202 by providing for information or software downloads to mobilestation 202 other than through a wireless communication network. Thealternate download path may, for example, be used to load an encryptionkey onto mobile station 202 through a direct and thus reliable andtrusted connection to thereby provide secure device communication.

Short-range communications subsystem 240 of FIG. 5 is an additionalcomponent which provides for communication between mobile station 202and different systems or devices, which need not necessarily be similardevices. For example, subsystem 240 may communicate with an acousticdevice (not shown) that may include an infrared device and associatedcircuits and components, or a Bluetooth™ communication module to providefor communication with similarly-enabled systems and devices. Bluetooth™is a registered trademark of Bluetooth SIG, Inc.

The above-described embodiments are meant to be illustrative ofpreferred embodiments and are not intended to limit the scope of thepresent method. Also, various modifications, which would be readilyapparent to one skilled in the art, are intended to be within the scopeof the present method. The only limitations to the scope of the presentapplication are set forth in the following claims.

1. In a mobile device having a first audio baseband circuit and a secondaudio baseband circuit and an interface between the first audio basebandcircuit and a second audio baseband circuit, a method for internalverification of the interface and first and second audio basebandcircuits comprising the steps of: a. setting the second audio basebandcircuit into a loopback mode; b. sending a test signal from the firstaudio baseband circuit to the second audio baseband circuit via theinterface while the second audio baseband circuit is in loopback mode;c. receiving at the first audio baseband circuit a second signal via theinterface, the second signal being the test signal sent to, and loopedback from, said second audio baseband circuit; and d. comparing thesecond signal received with an expected result corresponding to the testsignal sent, wherein successful internal voice path verification ispredicated based on the comparison.
 2. The method of claim 1, whereinthe interface is a code-decode digital interface.
 3. The method of claim2, wherein the code-decode digital interface is a pulse code modulationinterface.
 4. The method of claim 1, wherein said first audio basebandcircuit comprises a digital audio baseband chip of a radio frequencybaseband chip of the mobile device.
 5. The method of claim 1, whereinsaid second audio baseband circuit comprises a digital audio basebandchip of a Bluetooth™ baseband chip.
 6. The method of claim 5, whereinthe setting step involves setting the loopback mode in a digitalinterface module of the second digital audio baseband chip.
 7. Themethod of claim 5, wherein the setting step involves setting theloopback mode in an analog interface module of the Bluetooth™ basebandchip.
 8. The method of claim 1, wherein the first audio baseband circuithas a dual tone multiple frequency generator module associatedtherewith, the test signal being one of a dual tone multiple frequencytone and a single frequency tone.
 9. The method of claim 1, wherein saidcomparing step checks said second signal for an expected signal leveland an expected frequency.
 10. The method of claim 1, further comprisinggenerating the test signal at the first audio baseband circuit.